One method of achieving speed/angle compensation
is by utilizing the compressibility of air in the
pneumatic cylinders that activate the stabilizer.
As air pressure is applied to the cylinders, the fins
deflect until the pneumatic pressure inside the cylinders
is balanced by the hydrodynamic pressure on the fins
themselves. If the vessel's speed increases, the
hydrodynamic pressure also increases, catching the angle
of deflection of the fins to reduce as the air inside the
activating cylinders compresses.
At the same time, if the boat speed is reduced, the
hydrodynamic force on the fins also reduces, and
the air inside the cylinders expands with the
decreased pressure, yielding a greater angle of
fin deflection. This balancing of the hydrodynamic
and pneumatic pressures ensures that the lift force
produced by the fins remains almost constant regardless
of the intensity of the rolling. producing maximum
stabilization over a wide range of operational speeds.
Stress on the hull
Obviously, roll stabilizers stress the vessel's hull,
particularly at the fin locations, and the larger the
fins, the greater the stress. Yachts longer than about
50 feet, equipped with a single pair of large lins
located amidships, will also endure high twisting stresses.
This fact has led to the introduction of multiple
smaller fins, two or three per side, instead of the
single large pair of the same total area. For instance,
a 50-footer cruising at 10 knots requires 4.5 sq. ft. of
fin area on each side of the hull. This may be arranged
as a single fin or as two fins of 2.25 sq. ft. each. Not
only do smaller multiple fins reduce stress on the hull,
but they have also proved to be more efficient as stabilizers
and less obstructive for navigation. Moreover, if one fin is
damaged, the remaining three would produce 75 percent
stabilization, as compared with 50 percent when one of a
two-fin system is lost.
Traditionally, the stabilizing fins have been made
of fiberglass over a metal frame, attached to the
driving mechanism by a shaft passing through the
hull via a watertight fitting.
The shaft must carry both
the bending stresses exerted by the water pressure as
well as the torsional stresses created by the drive
mechanism. These combined stresses have dictated the
use of a large diameter stainless steel shaft that is
unlikely to break or bend if the fin strikes an object.
Because of the likelihood of hull damage in case of accident,
some fins are designed to retract upon collision, and several
breakaway models have recently been introduced. The most
intrigtling so far calls for the separation of bending and
torsional stresses, which is accomplished by installing the fin
on a stationary tubular post firmly attached to the outside of
the hull. Proportional deflection of the fin is achieved by a
solid shaft inside the tube and attached to the fin, which is
free to rotate orotund the post.
In this arrangement, the stationary post carries only bending
stressesa while the solid shaft bears only torsional stress.
The diameters of both shaft and tube are considerably reduced
from the single-shaft requirement and an impact with a submerged
object will easily shear the tube and bend the drive shaft. A
breakaway seal prevents seawater from reaching the interior of
the vessel if the fin is damaged.
Power supply for a pneumatic system is obtained from a compressor,
which is belt-driven or directly gear-driven off the main engine.
In some installations, an independent compressor and driving motor
are employed. Power supply for a hydraulic-electrical system is
obtained from a hydraulic pump which is belt-driven off the main
engine or independently driven from a separate electrical power
source.
Stabilizer systems may be installed in existing as well as new
vessels and the work generally takes three to five days. In
general, multiple-fin arrangements are simpler, as they do not
require hull reinforcement at the fin locations and the units
involved are lighter and easier to work with.
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